X-ray fluorescence analysis is a general designation of all techniques based on irradiating a target with X-rays, collecting fluorescent radiation induced in said target by said X-rays and drawing conclusions about the characteristics of said target on the basis of the intensity and frequency spectrum of the fluorescent radiation. Common applications of X-ray fluorescence analysis include investigating the material composition of the target. In many cases the main interest is to determine, how much the target contains some additive, pollutant or other minority constituent that can have important, potentially harmful effects, such as sulphur in liquid fuels or lead in paints. In other cases the analysis aims simply at determining the relative amounts of the main constituent materials, such as the different metals included in an alloy.
FIG. 1 is a schematic, partly cut-out diagram of a typical portable X-ray fluorescence analyzer device. Main parts thereof are a handheld measurement head 101 and a portable support unit 102 acting as a power source as well as a data processing and storage unit. If the processing and storage electronics can be made small enough, the whole analyzer device may even consist solely of the measurement head 101. The measurement head 101 comprises a handle 103 and a component compartment 104, which houses an X-ray source 105 and a detector 106. In order to suppress temperature-dependent interference there is a cooling arrangement 107 adapted to actively cool the detector 106. The cooling arrangement 107 includes typically a peltier element or a corresponding thermoelectric cooling device.
In order to perform an X-ray fluorescence analysis of a target, a front end 108 of the measurement head is pressed against a surface of the target. The X-ray source 105 emits incident X-rays towards the target through a window 109 located at the front end 108. The part of the window 109 that is transparent to incident X-rays and fluorescence radiation is typically made of (Kapton) polymide film or the like. Fluorescent X-rays that originate in the irradiated target enter the measurement head 101 through the window 109 and hit the detector 106.
If the surface of the target is hot, pressing the front end 108 against it will cause heat to be transferred to the body of the measurement head 101, which in turn gradually increases the internal temperature of the measurement head 101. The cooling arrangement 107 is only capable of maintaining some maximum temperature difference ΔTmax between the detector 106 and its surroundings. If the ambient temperature increases beyond a certain threshold, not even a maximum effort made by the cooling arrangement 107 will be sufficient to keep the detector 106 from reaching a cut-off temperature at which it will not be able to provide reliable results any more. The analyzer device typically includes a monitoring function, which will shut it off or at least produce an alarm if the temperature of the detector is not within some predetermined limits.
An obvious solution for improving the usability of an analyzer device for measuring hot surfaces would be to use a more effective cooling arrangement, and/or to extend its area of influence so that it will also cool the outer cover and other structural parts of the measurement head. However, such an obvious solution is unattractive especially from the viewpoint of power management. Thermoelectric cooling has relatively low efficiency (at least concerning the implementations known at the time of writing this description), which means that it draws a relatively high amount of electric power. In a portable analyzer device electric power comes from (rechargeable) batteries, so increasing power consumption will inevitably shorten battery life and thus decrease usability. Another obvious solution would be to make the outer cover of the measurement head from a material that has low thermal conductivity. However, the outer cover is there also for other purposes than for temperature shielding, and these other purposes may place other requirements to the material that are even contradictory to low thermal conductivity or at least require making compromises.
Yet another obvious solution would be not to press the end of the measurement head against the hot surface at all, but to keep it close to it at a small distance. This, however, involves at least two serious drawbacks. The measurement geometry will not be the same as in a press-against measurement, which will introduce a systematic error to the results. Additionally some X-ray radiation might escape into unwanted directions, which causes a small but not insignificant radiation hazard if the user of the device is not careful. Typically the front end of a measurement head includes a proximity sensor that prevents incident X-rays from being switched on if the front end is not pressed firmly against a surface of a target to be measured.